External Electrode Fluorescent Lamps (EEFL) are widely used in LCD panel backlight applications. Since the operational impedance of an EEFL has positive V-I characteristic, many individual EEFL lamp tubes may be connected in parallel, all of which may be driven by a single pair of inverter circuit. However, connecting multiple EEFLs directly in parallel presents difficulties for open/broken lamp detection in EEFL backlight system design since only the total current supplied to all the lamps in the panel, in aggregate, is sensed, rather than sensing the current of individual lamps in the system.
For example, in one conventional EEFL backlight system, open lamp detection is implemented by detecting the operational voltage of EEFL lamps. The total amount of current flow through all the lamps is controlled by a DC/AC inverter. If any lamp is broken, the total current will not change, and the rest of lamps will share the amount of current previously provided to the broken lamp. This results in lamp voltage increase in the remaining lamps due to increase of current in each of the remaining lamps. If the voltage is higher than a threshold, a DC/AC inverter judges an open lamp condition. Since the normal operation voltage of an EEFL lamp has at least +/−10% tolerance and also varies with temperature, this method is only able to detect if more than ¼ of total number of EEFLs become broken. For example, in a 20 EEFL backlight system, this conventional EEFL backlight system is unable to detect if less than 5 lamps are broken, since the lamp voltage change of less than 5 broken lamps is within the normal operation voltage tolerance range.